1. Field of the Invention
The present invention relates to a semiconductor laser device having a carrier-confining structure for confining the two types of carriers: electrons and positive holes.
2. Description of the Prior Art
In recent years, a semiconductor substrate with a growth plane other than the (100) plane has been employed for the growth of semiconductor crystals thereon by the use of molecular beam epitaxy. It has been found that silicon (Si), which serves as an n-type impurity in an AlGaAs layer grown on the (100) plane of a GaAs substrate, functions as a p-type impurity in an AlGaAs layer grown on the (n11) plane (where n is an integer equal to or greater than 1) of the GaAs substrate (Appl. Phys. Lett., 47, 826 (1985)). The cause for such inversion of the conductivity type is considered to be as follows: When a III-V group compound semiconductor layer (e.g., an AlGaAs layer such as mentioned above) is grown on the (n11) growth plane, the V group element of the III-V group compound semiconductor layer to be grown has a low sticking coefficient. Thus, when Si, a IV group amphoteric element, is added thereto as an impurity, it is readily taken into the lattice points for the V group element in the III-V group compound semiconductor layer. This causes the Si to function as an acceptor, i.e., a p-type impurity.
By utilizing the above-described phenomenon, both a current-confining structure and a refractive index waveguide structure can be formed in a single growth step using molecular beam epitaxy in the production of a semiconductor laser device (Japanese Laidopen Patent Publication No. 1-109789). A conventional semiconductor laser device of this type is shown in FIG. 2, which is produced, for example, as follows:
First, as shown in FIG. 2, a p-type GaAs substrate 30 with a ridge-type mesa 32 thereon is so prepared that the ridge-type mesa 32 has the (n11)A planes (where n is an integer equal to or greater than 1) as its inclined faces 31. On the surface of the p-type GaAs substrate 30 provided with the ridge-type mesa 32, a p-type AlGaAs first cladding layer 33, a non-doped GaAs active layer 34, and an n-type AlGaAs second cladding layer 35 are successively grown by molecular beam epitaxy. For the growth of the n-type AlGaAs second cladding layer 35, a IV group amphoteric element is added as an impurity. In the inclined portions of the n-type AlGaAs second cladding layer 35 located above the inclined faces 31 of the ridge-type mesa 32, the IV group amphoteric element added as an n-type impurity functions as a p-type impurity, so that these inclined portions become a p-type inversion layer 36. Other layers such as a contact layer and insulating film to be formed further on the n-type AlGaAs second cladding layer 35, and p-sided and n-sided electrodes are not shown in FIG. 2, and the description thereof is also omitted.
In the above-described conventional semiconductor laser device, current is confined by the p-type inversion layer 36.
However, in this semiconductor laser device, as shown in FIG. 2, reactive current flows through the p-type GaAs substrate 30, then through the p-type AlGaAs first cladding layer 33, then through the inclined portions 38 of the non-doped GaAs active layer 34 located under the p-type inversion layer 36, and then through the p-type inversion layer 36, and thereafter through a portion 37 of the n-type AlGaAs second cladding layer 35 located above the top face of the ridge-type mesa 32. Furthermore, in this semiconductor laser device, electrons with high mobility are confined by the p-type inversion layer 36, but positive holes with low mobility are not confined. This further increases the reactive current in the semiconductor laser device.